The close links between a private sector lobby group and an NHS regulator in the runup to the launch of a groundbreaking inquiry into competition in the health service have emerged in a series of documents passed to the Guardian.

Emails released under the Freedom of Information Act show that NHS Partners Network, a lobby group which represents companies including Care UK, Circle, General Healthcare Group, Bupa and United Health, helped draft a letter requesting a formal investigation into how firms were being blocked from getting NHS work. A week later the private healthcare lobby group took the regulator out to a £250-a-head gala dinner.

The network began lobbying the Co-operation and Competition Panel (CCP) last October for an inquiry into restrictions on the use of non-state companies in the health service – an investigation that was given the go-ahead two months later. The result was a report published this week which included recommendations to offer patients "more choice" because people were "dying" while waiting for operations in NHS hospitals.

Since 2006, patients have had a right to choose where they go for treatment for elective surgery, including private hospitals. In this week's landmark ruling the panel found that almost half of NHS primary care trusts, the state bodies which control health budgets, were unreasonably restricting patients' choice over where they go for operations.

The tactics employed included setting minimum waiting times before patients were getting treated – even when private providers could treat them – and directing GPs to refer patients to keep cash flowing into a local hospital.

In a contentious passage the regulator claimed that "we understand that patients will 'remove themselves from the waiting list' either by dying or by paying for their own treatment at private sector providers".

Andrew Lansley, the health secretary, leapt on the findings, telling the BBC that "too many [trusts] have been operating in a cynical environment where they can game the system – and in which political targets, particularly the maximum 18-week waiting time target, are used to delay treatment."

However, David Stout, director of the PCT Network, described this claim as "unsubstantiated", pointing out that average waiting times were just eight weeks for what were non-emergency and therefore non-life threatening operations. He said the regulator's statements "cause unnecessary public anxiety and alarm".

Emails, obtained by Spinwatch which campaigns for greater transparency in government, between the head of the CCP, Andrew Taylor, and David Worskett, the director of the NHS Partners Network, reveal the inquiry followed a letter from the lobby group to the Department of Health, which Taylor helped draft.

The letter sent on 8 November last year to Bob Ricketts, director of provider policy at the Department of Health, proposes the department uses its powers to commission an inquiry into concerns it has about "maverick behaviours" by NHS commissioners in health trusts. "It might be helpful to ask the CCP to carry out a sector inquiry," it says.

The CCP, which advises the government on anti-competitive practices, describes itself as "an independent, transparent and effective advisory body … committed to using fair and transparent processes".

Documents show Taylor being asked by the lobby group: "What do you think of the draft letter?" Taylor replies with revisions to the suggested scope of the inquiry, adding: "Looks good to me."

The lobby group stated it did not want to show its hand in public, asking Taylor to keep details secret "until we actually press the 'go' button". An email to Taylor from the Network reads: "I have discussed the approach with [Jill Watts, CEO of private company Ramsay Health], my chair, but so far with no one else and I would prefer to keep it fairly tight."

In another frank email in October last year, Worskett recalls a meeting with health minister Simon Burns: "We actually saw Burns last Thursday … he was dreadful … independent sector leaders are very unhappy."

In the same week that Taylor helped draft the industry's letter proposing the inquiry, he and a CCP colleague were invited to join the lobby group's table at the £250-a-head Health Investor Power 50 awards gala dinner. Watts received the award for the most influential person in the private sector.

Wendy Savage, of Keep our NHS Public, said that campaigners against privatisation and competition could "not afford to take people out at the drop of a hat. We don't have the money to run these types of lobbying operations".

Further discussions between the CCP and the lobby group in May this year reveal industry concerns over how to beef up the CCP to ensure the government acts on its recommendations. Taylor emails suggestions for making the CCP "more effective", including requiring the secretary of state to respond to its recommendations within a 30-day timeframe: "That would make sure that CCP recommendations could not be ignored," he wrote.

The Department of Health said: "What's important is that the Co-operation and Competition Panel carries out its duties – to ensure that there is choice, co-operation and that the NHS is not restricting competition to the detriment of patient care. We are confident that the CCP fulfils those duties effectively."

Worskett said: "The tone of correspondence between the NHS Partners Network and the CCP is open and frank, as we believe it should be in all our dealings. In any event, tone is less important than the need for the CCP to consider the issues being raised fairly. For our members, that is concern that some parts of the NHS are restricting competition to the detriment of patient care."

The CCP denied it had been improperly influenced, arguing that it was an "impartial body that takes in evidence from all parties and advises equitably on competition issues in the NHS".